Within the packaging industry, it is well known to employ laminated packaging materials of a single-use disposable nature for the packing and transport of liquid foods. Normally, such laminated packaging material is built up from a configurationally rigid but foldable core layer, for example consisting of paper or paperboard, in order to obtain good mechanical configurational stability. Liquid-tight coatings of plastic are laid on both sides of the core layer and protect the liquid-absorbing fibres of the core layer effectively from being penetrated by moisture. These outer layers normally consist of a thermoplastic, preferably polyethylene, which moreover imparts to the packaging material superior thermosealing properties so that the packaging material may be converted into finished packages of the desired geometric configuration.
However, laminated packaging material which consists solely of paper or paperboard and liquid-tight plastic lacks tightness against gases, in particular oxygen gas. This is a major disadvantage in the packing of many foods whose shelf-life declines dramatically when they come into contact with oxygen gas, for example fruit juices. In order to supplement the packaging material with a barrier against gases, in particular against oxygen gas, it belongs to the prior art technology to lay on a layer possessing superior tightness to oxygen gas, for example aluminium foil or polyvinyl alcohol, on that side of the core layer which is intended to face in towards the interior of the package.
In comparison with aluminium foil, polyvinyl alcohol possesses many desirable properties, for which reason it is to be preferred as a barrier material in many contexts. Among other things, polyvinyl alcohol possesses higher mechanical strength, better compatibility with foods and is more economical, at the same time as enjoying excellent properties as an oxygen gas barrier. It has further been deemed as a suitable material, in certain cases from the environmental viewpoint or with a view to recycling and recovery to replace aluminium foil as gas barrier material in food packages.
One drawback is that polyvinyl alcohol is sensitive to moisture and rapidly loses its barrier properties when it is exposed to a damp environment. This drawback was previously obviated according to WO97/22536 in that the polyvinyl alcohol was combined with one or more per se known, food approved polymers, for example copolymers ethylene/acrylic acid (EAA) or copolymers of styrene/butadiene. These form, in combination with polyvinyl alcohol, a continuous and unitary layer with superior properties as gas barrier, in particular against oxygen gas, at the same time as the desired superior gas barrier properties of the polyvinyl alcohol are retained also in a damp environment.
While polymer gas barrier materials may impart good gas barrier properties to a packaging laminate, they are nevertheless to some extent permeable for oxygen gas, while a material of a type such as metal or glass for cans or bottles possesses an oxygen gas permeability which is practically zero. In order further to improve the gas barrier properties, it is possible to mix the polymer gas barrier material with an inorganic lamellar material. Such a polymer composition for gas barrier material is described, for example, in EP-A-O 590 263 and possesses excellent barrier properties against gas and moisture. Thus, EP-A-O 590 263 presents a method for the production of a polymer composition for a gas barrier material, or the formed product, e.g. a film, the composition including a polymer and an inorganic lamellar material with a particle size of 5 μm or less and a quantity relationship of 50-5000, and the method comprises the step that the inorganic laminate material is dispersed in a polymer or a polymer solution in such a state that the inorganic lamellar material swells or shears up in a solvent/dispersion agent, whereafter this is removed from the dispersion when necessary in the form of a film, at the same time as the lamellar material remains in the swollen state.
From JP 56004563 is also known a plastic material which, with a view to attaining superior gas and moisture barrier properties, has been coated with a film of polyvinyl alcohol (PVOH) and SiO2. The relationship SiO2/PVOH may be 5/95-80/20. It is also disclosed that the size of the SiO2 particles does not constitute a limitation but that they should have an average diameter of below 100 micron. It is further disclosed that an aqueous dispersion which contains PVOH and SiO2 is utilised for the coating and that thermocuring may be carried out, for example during a period of time of up to an hour in a temperature of 80-200° C.
According to JP10-001515, the intention is to crosslink the main chains in PVOH by means of SiO2. The purpose is, in a laminate with a core layer of paperboard, to improve gas barrier properties despite a damp and humid environment.
WO 99/44826 describes a laminate with a core layer of biaxially oriented polypropylene. An outer layer may consist of a homo- or copolymer of a vinyl alcohol which may include an “antiblocking agent”. Such an “antiblocking agent” may consist of spherical SiO2 particles of the order of magnitude of 1-6 μm, in a quantity of 0.1-2 weight %.
EP 761 876 describes a packaging laminate for, for example, a package for liquid foods, the laminate being built up with a core of paperboard. A coating layer on the paperboard, the coating layer including PVOH and amorphous SiO2 has for its purpose to act as a moisture and aroma barrier.
WO 00/40404 describes a thermoplastic film intended for the packing of food products, e.g. by wrapping them in a transparent film, but not intended for the production of dimensionally stable packages for liquid foods. The film according to WO 00/40404 has a coating layer on at least one surface thereof, the coating layer including a polymer binder or adhesive and an additive including nanoparticles of, for example, SiO2. It is disclosed that the nanoparticles preferably constitute 5 to 20 weight % of the additive, and that the additive constitutes 40 to 90 weight % of the coating.